Pressure-diecasting, using for example, the so-called Zamak alloys available from the New Jersey Zinc Company, which comprise zinc, with aluminium, copper and magnesium as alloying additions, has been regarded, for a comparatively long time, as a successful economical process in the mass production of small parts for electromechanical components, especially in the case of plug-and-socket connectors and potentiometers. Parts manufactured from Zamak alloys are resistant to the atmospheric corrosion effects which are normally encountered. The natural grey layer which forms on the parts prevents the corrosion from becoming progressive. If a decorative effect is to be achieved, such parts can also be subjected to a subsequent surface-finishing process. Zamak alloys parts are usually plated with copper, nickel, chromium, cadmium or tin. With regard to the quality of the castings, and especially with regard to those of their characteristics which are relevant to production, and with regard to the quality of their surface finish, particular importance attaches to the die temperature. In addition to the die temperature, the melt temperature should also be continuously monitored. Fluctuating melt and die temperatures lead to castings of variable quality. Furthermore, the quality of the parts is influenced by a series of additional casting parameters which relate to details of the machine and casting system employed (for example, the velocity of the melt at the casting gate, dissimilar cooling conditions following the removal of the parts from the die, die-filling conditions, etc.). The machine-related casting parameters also include the influence of the liquid metal level. Comprehensive tests have shown that, for a fixed setting of the change-over point, the liquid metal level exerts an influence on the position of the melt relative to the casting gate. In the case of a first level setting, the melt stood immediately in front of the gate as the die-filling phase was initiated. The velocity of the casting piston remained constant throughout the die-filling phase, as did also the pressure to which the melt in the casting system was subjected as the die was filled. In the case of a second level setting, the liquid metal level had fallen, so that the position of the change-over point, from the first phase to the second phase, was such that the melt stood a considerable distance in front of the gate. The velocity of the casting piston decreased when the piston reached the melt in the gate, and a high pressure-peak occurred concurrently with the decrease in the velocity. In the case of a third level setting, the liquid metal level had been raised. This led to a condition in which a portion of the die cavity was already filled during the first casting phase. On this occasion, the velocity of the casting piston increased continuously as the second phase was initiated. Only in the second half of the die-filling phase did the velocity of the casting piston reach a constant value. With the increase in the velocity, the pressure in the casting system, to which the melt was subjected during the die-filling phase, also increased, but scarcely any pressure-peak could be recognised. The occurrence of pressure-peaks during the die-filling phase must be taken into account with regard to the stressing of the pressure-casting die which forms part of the closing system of the pressure-diecasting machine, the occurrence of these pressure-peaks being dependent on the setting of the pressure-diecasting machine, the position of the change-over point, and the liquid metal level. High pressure-peaks can lead to the occurrence of fatigue fractures in the columns in the closing mechanism of the pressure-decreasing machine.
From the above considerations, it is evident that the liquid metal level represents an important casting parameter, and that efforts should be made to minimise the fluctuations to which it is subject. Hitherto, the charging of the holding vessel, which can be heated and is filled with the liquid metal melt, was carried out in a manner according to which the machine operator had to decide, by visually assessing the liquid metal level, whether or not it was necessary to replenish the melt with a new pig. The level typically fluctuated by .+-.2.5 cm.